(1) Field of the Invention
The present invention generally relates to a method for making solar cell, and more particularly to a surface texturization process toward a silicon wafer of the solar cell.
(2) Description of the Prior Art
Typical method for making solar cell usually comprises a surface texturization process to enhance texturized (or roughness) level of the surface of the solar cell. The surface texturization process is generally for enlarging the surface square measure of the solar cell to increase the absorbed light and to raise the photo-electricity conversion efficiency.
Prior solar cell made from monocrystalline silicon is usually texturized by utilizing alkaline solution such as potassium hydroxide (KOH) solution or sodium hydroxide (NaOH) solution. Anisotropic etching is proceeded on the solar cell's surface by using these alkaline solutions. However, while applying such method to the solar cell made from multicrystalline silicon, several problems arise:
1. Because the original crystal surface of multicrystalline silicon is not <100> orientation surface, Pyramid like micro-structure will not be formed on the surface after etching. This is unfavorable for light absorbing.
2. Because the original surface of multicrystalline silicon comprises various orientations of crystalline grain having different etching rate. Somehow, fault structures between these different orientations of crystalline grain will be formed after etching. As a result, the fault structure would result in electrically disconnection, or resistance increasing of solar cell interconnection.
To resolve the mentioned drawbacks, surface texturization process utilizing isotropic etching is adopted. This kind of surface texturization process is substantially divided into two category: 1) Dry etching; and 2) Utilizing acidic solution for etching. The present invention generally relates to the latter one. About utilizing acidic solution for etching, there are several typical prior arts:
1. Please refer to patents occupied by Mitsubishi Denki Kabushiki Kaisha (Tokyo, Japan): U.S. Pat. Nos. 6,156,968, 6,391,145, 6,340,640 and JP10-303443. These patents generally disclose utilizing mixed solution of nitric acid, hydrofluoric acid and at least one “carboxylic acid, which has a molecular weight more weighting than acetic acid” and “mixed solution of phosphoric ace and carboxylic acid having a molecular weight more weighting than acetic acid” for surface texturization process. After etching, diluted sodium hydroxide is needed for removing the formed compound from the process. Within these patents, the U.S. Pat. No. 6,156,968 relates to the etching structured of the solar cell; the U.S. Pat. No. 6,340,640 and the JP10-303443 relate to the etching process; and the U.S. Pat. No. 6,391,145 relates to the apparatus for the etching process.
2. Please refer to patents occupied by Photowatt International S.A. (Bourgoin-Jallieu, France): U.S. Pat. No. 5,646,123 and JP9-167850. These patents generally disclose utilizing a two-stages process to texturize the surface of solar cell made from p-typed poly silicon. The first stage is oxidizing the surface of solar cell by an oxidizing solution containing fluorine ion to make the surface a porous layer. The second stage is dissolving the oxidized porous layer by a dissolving solution to reveal a texturized surface cratered with cavities of depth and diameter lying in the range of 0.10 .mu.m to 10 .mu.m. The oxidizing solution in the first stage is selected from the chromate ion, the bromate ion, the nitrate ion, the nitrite ion, the amine ion, a metallic redox couple, and mixtures thereof. The dissolving solution in the second stage is an alkaline solution.
3. Please refer to patents occupied by SHINRYO CORP (Japan): JP2004-063744. The patent discloses an etching method has a first process for immersing the unprocessed silicon substrate in mixed acid water solution A comprising 36 weight % to 42 weight % of hydrofluoric acid and 6 weight % to 10 weight % of nitric acid and etching the substrate, and a second process for immersing the silicon substrate etched in the first process in mixed acid water solution B comprising 42 weight % to 46 weight % of hydrofluoric acid and 2 weight % to 6 weight % of nitric acid and etching the substrate. The first process is for forming a plurality of convex-concave structure on the solar cell's surface. The second process is for forming a plurality of micro convex-concave structure on the convex-concave structure. The mixed acid water solution A further comprises 1 weight % to 10 weight % acetic acid. The mixed acid water solution B further comprises 1 weight % to 5 weight % acetic acid.
4. Please refer to patents occupied by Mitsubishi Denki Kabushiki Kaisha (Tokyo, Japan): JP2000-323736. The patent discloses forming a Saw damage layer on the solar cell by utilizing isotropic etching solution, then utilizing spin etcher and mixed solution of nitric acid and hydrofluoric acid to from convex-concave structures.
The listed typical prior arts are all belonging to the category of utilizing acidic solution for etching. Almost all of them require hydrofluoric acid. Although these prior arts is capable of providing usable surface texturized structure, however, the process comprising using hydrofluoric acid is extremely dangerous, hazardous for environment and difficult to management. In anther aspect, these prior arts need alkaline solution to remove the formed compound from etching, or they have to divide into two stages. This is another inconvenience.